An engine is powered by burning of fuel such as gasoline and utilizing the produced power. In a normal 4-cycle engine, four strokes of intake, compression, expansion (combustion) and exhaust are one cycle and repeated.
An increase in the thermal efficiency of the engine is effective in improving the fuel efficiency or exhaust gas temperature and thereby enhances the catalytic activity. Accordingly, efforts to increase thermal efficiency of an engine are still continuing at present.
In order to increase the thermal efficiency of an engine, retaining of heat during combustion may be first considered. To realize this, the temperature in the combustion chamber is preferably high in the expansion (combustion) stroke. In this case, the property required of the wall surface of the combustion chamber is low thermal conductivity, i.e., high thermal insulation property. As for the thermal insulation technique that has been heretofore studied, an engine in which a ceramic coating is applied or the combustion chamber itself is composed of ceramic, while forming an air layer on the back of the chamber, and thermal insulation is thereby achieved is known. This technique is characterized in that the heat loss from the combustion chamber to cooling water is reduced by causing the wall surface to act as a thermal barrier and the energy is recovered by piston work or a turbo charger so as to enhance the thermal efficiency.
However, if the thermal insulation property is excessively enhanced, the wall temperature of the combustion chamber increases the operating gas heat and this causes impairment of the intake efficiency and an increase of NOx emissions. Furthermore, a high temperature heat-shielding layer disadvantageously results in a problem of lubricity.
To overcome this problem, a heat-shielding technique causing no rise in the wall temperature of the combustion chamber is required in the intake stroke. Specifically, this is a technique where, as the material characteristics, a heat-shielding film having low thermal conductivity and low heat capacity is formed on the wall surface of the combustion chamber and the wall surface temperature is varied according to the gas temperature (a low temperature during intake and a high temperature during combustion), whereby the temperature difference between the combustion gas and the wall surface is reduced and prevention of intake air heating and reduction of heat loss are simultaneously attained.
Non-Patent Document 1 (Victor W. Wong, et al., Assessment of Thin Thermal Barrier Coatings for I.C. Engines, Society of Automobile Engineers, Document Number: 950980, Sate Published: February 1995) describes a technique where a thin-film material having low thermal conductivity and low heat capacity is formed on the wall surface of the combustion chamber so as to simultaneously attain the reduction of heat loss and the prevention of intake gas overheating based on the above. A sprayed film of ZrO2 is described as a specific thin-film material. However, the sprayed film of ZrO2 readily causes separation or drop-off, and durability/reliability being insufficient remains.
Meanwhile, with the recent increase in engine power, the temperature in the combustion chamber becomes high and the local heat load tends to rise in the combustion chamber, which may lead to generation of thermal strain or cracking in the member constituting the combustion chamber.
For reducing such thermal strain, Patent Document 1 (Kokai (Japanese Unexamined Patent Publication) No. 2003-113737, description) describes a technique of forming a porous ceramic layer by anodic oxidation on a cylinder head constituting a part of the combustion chamber and thereby reducing thermal conduction from the combustion chamber to the cylinder head.
Also, for reducing the cracking, Patent Document 2 (Kokai No. 1-43145, description) describes a technique of forming an alumite layer by anodic oxidation on a piston top constituting a part of the combustion chamber, and further forming a ceramic layer by spraying, thereby reducing thermal conduction from the combustion chamber to the piston top.
As described above, Patent Documents 1 and 2 are intended to achieve reduction of thermal conduction. However, when only thermal conduction is reduced, the wall temperature of the combustion chamber rises causing intake gas overheating and there remains a problem that an impairment of the intake efficiency and an increase of the NOx emissions are incurred.